Groundwater-related subsidence
Groundwater-related subsidence is the subsidence
Subsidence is the motion of a surface as it shifts downward relative to a datum such as sea-level. The opposite of subsidence is uplift, which results in an increase in elevation...

 (or the sinking) of land resulting from groundwater
Groundwater is water located beneath the ground surface in soil pore spaces and in the fractures of rock formations. A unit of rock or an unconsolidated deposit is called an aquifer when it can yield a usable quantity of water. The depth at which soil pore spaces or fractures and voids in rock...

 extraction, and a major problem in the developing world as major metropolises swell without adequate regulation and enforcement, as well as a being a common problem in the developed world. One estimate has 80% of serious land subsidence problems associated with the excessive extraction of groundwater, making it a growing problem throughout the world.

Groundwater is considered to be one of the last 'free' resources. Anybody who can afford to drill, can draw up merely according to the their ability to pump. However, as seen in the figure, the act of pumping draws down
Drawdown (hydrology)
In water-related science and engineering there are two similar but distinct definitions in use for drawdown.*In subsurface hydrogeology, drawdown is the change in hydraulic head observed at a well in an aquifer, typically due to pumping a well as part of an aquifer test or well test.*In surface...

 the free surface of the groundwater table, and can affect a large region. Thus, the extraction of groundwater becomes a Tragedy of the commons
Tragedy of the commons
The tragedy of the commons is a dilemma arising from the situation in which multiple individuals, acting independently and rationally consulting their own self-interest, will ultimately deplete a shared limited resource, even when it is clear that it is not in anyone's long-term interest for this...

, with high economic externalities.

The desert
A desert is a landscape or region that receives an extremely low amount of precipitation, less than enough to support growth of most plants. Most deserts have an average annual precipitation of less than...

 areas of the world are requiring more and more water for growing populations, and agriculture
Agriculture is the cultivation of animals, plants, fungi and other life forms for food, fiber, and other products used to sustain life. Agriculture was the key implement in the rise of sedentary human civilization, whereby farming of domesticated species created food surpluses that nurtured the...

. In the San Joaquin Valley
San Joaquin Valley
The San Joaquin Valley is the area of the Central Valley of California that lies south of the Sacramento – San Joaquin River Delta in Stockton...

 of the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, groundwater pumping for crops has gone on for generations. This has resulted in the entire valley sinking an extraordinary amount, as shown in the figure. It has been argued that there is little consequence to subsidence in a wide, flat, agricultural basin, since the settlement is uniform. However, no large-scale change due to subsidence of the magnitude seen in the San Joaquin Valley is likely to be without some negative impact, since the hydrology of the area is now greatly changed. Other regions of the world, such as New Orleans, Louisiana, in the United States and Bangkok in Thailand are now subject to severe flooding due to subsidence associated with groundwater removal. Total settlement, and therefore the potential impacts of the settlement, can only be determined by surveys
See Also: Public Land Survey SystemSurveying or land surveying is the technique, profession, and science of accurately determining the terrestrial or three-dimensional position of points and the distances and angles between them...

 and GPS measurements.
Unfortunately, not all groundwater-related subsidence is benign. In Mexico City the buildings interact with the settlement, and cause cracking, tilting, and other major damage In many places, large sinkholes open up, as well as surface cavities. Damage from Hurricane Katrina
Hurricane Katrina
Hurricane Katrina of the 2005 Atlantic hurricane season was a powerful Atlantic hurricane. It is the costliest natural disaster, as well as one of the five deadliest hurricanes, in the history of the United States. Among recorded Atlantic hurricanes, it was the sixth strongest overall...

 was exacerbated due to coastal sinking, associated with groundwater withdrawal.

The cause of the long-term surface changes associated with this phenomenon, are fairly well known. As shown in the USGS figure, aquifers are frequently associated with compressible layers of silt
Silt is granular material of a size somewhere between sand and clay whose mineral origin is quartz and feldspar. Silt may occur as a soil or as suspended sediment in a surface water body...

 or clay
Clay is a general term including many combinations of one or more clay minerals with traces of metal oxides and organic matter. Geologic clay deposits are mostly composed of phyllosilicate minerals containing variable amounts of water trapped in the mineral structure.- Formation :Clay minerals...

As the groundwater is pumped out, the effective stress
Effective stress
Karl von Terzaghi first proposed the relationship for effective stress in 1936. For him, the term ‘effective’ meant the calculated stress that was effective in moving soil, or causing displacements...

 changes, precipitating consolidation, which is non-reversible. Thus, the total volume of the silts and clays is reduced, resulting in the lowering of the surface. The damage at the surface is much greater if there is differential settlement, or large-scale features, such as sinkholes.

The only known method to prevent this condition is by pumping less groundwater, which is extremely difficult to enforce when many people own water wells. Attempts are being made to directly recharge aquifers but this still a preliminary effort.
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